Various materials for forming a transparent conductive layer and various transparent conductive layer films obtained by forming a transparent conductive layer on a transparent substrate are used as important functional components in, for example, electronic devices that utilize a light emitting/receiving function. In particular, components to which a function of an electrode, a switch, or the like is provided by arranging a large number of conductive areas on a transparent substrate by patterning a transparent conductive layer are essential components for realizing reduction in the thickness and size of such electronic devices, and for realizing high performance of such electronic devices.
Hitherto, an ITO film, which is composed of an indium-based oxide, has been mainly used as a transparent conductive layer because of its high transmittance of visible light, low surface electrical resistance, and excellent environmental characteristics. There are various methods for producing an ITO film functioning as a transparent conductive layer. Among these methods, a main method is a sputtering method, in which a dilute inert gas is introduced in a vacuum, and inert gas ions generated by DC or RF discharge are accelerated and made to collide on a surface of an ITO target material so as to sputter atoms and molecules constituting the target from the surface, thus forming an ITO film on a surface by deposition. The sputtering method is advantageous in that a conductive layer having a low surface electrical resistance can be formed even on a somewhat large area. However, the deposition rate in the sputtering method is low, and in order to deposit a conductive film having a homogeneous quality, it is necessary to increase the accuracy of the control of an apparatus, for example, the gas concentration and the temperature inside the apparatus. For these reasons, it is difficult to increase the size of the apparatus.
For a transparent conductive layer formed of an ITO film or the like obtained by the sputtering method, a transparent conductive layer pattern is formed by an etching method. The etching method is performed by utilizing photolithography including steps such as application of a resist film, exposure, development, chemical etching, and removal of the resist film in a solution, the steps being sequentially performed on the transparent conductive layer. Thus, this process speed of the etching method is low, and the etching method has a problem in terms of a high production cost including the cost of disposal of liquid waste generated in the steps of development, chemical etching, and removal of the resist film by a wet process.
Furthermore, in order to obtain a dense, low-resistance, and transparent coating film using an ITO film formed by the sputtering method, the film must be baked at about 300° C., and thus it is impossible to form such a coating film on a plastic film.
As described above, the production of a transparent conductive layer requires a plurality of complicated steps in the film-forming process or the patterning process. Accordingly, it is difficult to significantly improve the production efficiency, and the suppression and reduction in the production cost are also limited. Furthermore, the type of substrate that can be used is also significantly limited because of the necessity of the baking.
A relatively low-cost method for obtaining a transparent conductive layer pattern using a printing method, the method having been hitherto employed, is a method for forming a transparent conductive layer pattern on a substrate using a conductive layer coating material prepared by dispersing conductive fine particles such as ITO particles in a binder solution and using a patterning method that utilizes a printing technology such as a screen printing method. This method is advantageous in that the apparatus is simple and productivity is high, and that a transparent conductive layer pattern having a large area can be produced at a low cost by only a coating step, as compared with the method in which a conductive film is formed by sputtering and is then patterned by etching. However, a conductive layer obtained by this method has a drawback of high electrical resistance because, in this method, a conductive coating film is formed using a binder resin and thus the contact between conductive fine particles becomes insufficient.
In another known method, a transparent conductive layer pattern is formed by preparing a binder-resin-free conductive coating material containing conductive fine particles such as ITO fine particles, a solvent, a coupling agent, and an organic acid salt or inorganic acid salt of a metal, applying the conductive coating material onto a substrate so as to form a pattern by a printing method such as a screen printing method, and conducting baking at a temperature of 300° C. or higher. In this method, since no binder is contained, the electrical resistance of the conductive layer decreases. However, since a high-temperature baking at 300° C. or higher is necessary for the film formation, it is difficult to form a conductive layer pattern on a flexible substrate such as a resin film.
Furthermore, as a method for forming a transparent conductive layer pattern having a low electrical resistance on a substrate having poor heat resistance using a simple, low-cost method for forming a transparent conductive layer without using a sputtering method, a method has been proposed in which a conductive layer pattern can be formed by using a conductive film for transfer, the conductive film including a support and a conductive layer provided on and detachable from the support, without conducting firing or baking at a high temperature.
For example, the following method for forming a conductive layer pattern has been proposed. The method includes preparing a conductive film for transfer, the conductive film including a support and a transparent conductive layer that is provided on and detachable from the support and that is formed of a compressed layer of conductive fine particles; bonding the transparent conductive layer to a surface of a substrate from the conductive film for transfer with an adhesive layer therebetween, the adhesive layer having an adhesive area patterned by exposure or the like; and detaching the support from the substrate so that only a portion of the conductive layer, the portion being in close contact with the adhesive layer in the adhesive area, is left on the substrate (refer to Patent Literature 1). It is described that the adhesive layer that bonds the transparent conductive layer to the substrate may be formed on the transparent conductive layer or may be formed on the substrate in advance. In the above method, the conductive layer is formed and fixed without performing a vacuum process for film formation, and firing or baking at a high temperature, and a pattern is formed without performing a wet process such as etching. However, in order to fix the transparent conductive layer, which is originally formed so as to be detached and which has a weak adhesive strength to the support and the substrate, the transparent conductive layer pattern is fixed on the substrate with the adhesive layer having a relatively large thickness therebetween. Consequently, the transparent conductive pattern including the adhesive layer is formed as a projecting layer having a large thickness. As a result, the conductive pattern, which should be transparent, easily visually recognized. When such a conductive pattern is used as a transparent conductive electrode for a touch panel or electronic paper, the transparent conductive electrode being used by bonding on a liquid crystal display panel, the visible electrode pattern adversely affects the quality of a display image.
A method for forming a patterned conductive layer on a substrate without interposing an adhesive layer has been proposed (refer to Patent Literature 2). Specifically, in this disclosed method, a desired pattern is formed by forming a photosensitive resin layer on a surface of a metal layer provided on a transparent substrate, partially forming an area having a strong adhesive strength on the photosensitive resin layer by light irradiation, and detaching, from the transparent substrate, a portion of the metal layer located at a position corresponding to the above area so as to left a necessary conductive layer pattern on the substrate.
However, in the above method, after the formation of the conductive pattern, the conductive layer is still in a state of being detachable from the substrate, and thus weak adhesiveness remains. Therefore, this method is not preferable from the standpoint of processing and practical use. Furthermore, in the previous step, the photosensitive resin layer is uniformly formed on the surface of the conductive layer, and the whole conductive layer contacts the photosensitive resin. Accordingly, a non-irradiated portion may remain on the conductive layer, or a desired necessary portion of the conductive layer may be detached from the substrate. If the conductive layer is a transparent conductive layer, such a remaining non-irradiated photosensitive resin may decrease the light transmittance of the transparent conductive layer. In addition, in the case where the adhesive strength of the conductive layer before detachment, the conductive layer being formed on the substrate, is weak, a conductive layer portion that should not be detached may also be detached from the substrate by the non-irradiated photosensitive resin. Accordingly, it is believed that the above method can be suitably used only when the detachable conductive layer formed on the substrate has an appropriate adhesive strength, the conductive layer and the substrate are not transparent, and the photosensitive resin remaining on the substrate is not visible either directly or through the substrate and the transparent conductive layer.
Meanwhile, a transparent conductive coating material capable of providing, as a novel transparent conductive layer, a coating film having high transparency and an electrical resistance that is as low as that of an existing ITO film, and a method for forming a transparent conductive layer pattern using the transparent conductive coating material have been proposed (refer to Patent Literature 3). This literature describes that, by using conductive nanowires each having a high aspect ratio of more than 10 and a cross-sectional dimension of less than 100 nm, a substantially transparent conductive wire network can be formed to form a transparent conductive layer. Furthermore, a method for forming a transparent conductive layer has been proposed in which a transparent conductive coating material prepared by dispersing, as such conductive nanowires, for example, silver nanowires produced by a specific method in a solvent is applied onto a substrate and dried, thereby arranging the silver nanowires so as to have a network shape and obtaining a transparent conductive layer having good transparency and conductivity.
Furthermore, as a method for forming a transparent conductive layer pattern using the above silver nanowires, described is a method for forming a transparent conductive pattern, the method including forming a conductive layer containing silver nanowires on a substrate, then fixing the silver nanowires so as to have a pattern using a binder resin or the like, and then washing or brushing a non-fixed area with an appropriate solvent or removing the area with an adhesive roller.
Also described is a method for forming a transparent conductive pattern, the method including forming a transparent conductive layer of silver nanowires on a substrate, then applying a coating material for fixing, the coating material being curable by light or heat, over the entire surface of the conductive layer, curing the coating material by applying light or heat only to a portion to be left as a pattern, and then removing an unnecessary portion by the same method as the method described above.
In the above methods, it is possible to form a transparent conductive pattern that has conductivity and high transparency and that is not easily visible. However, the conductive layer composed of the silver nanowires before fixing has a weak adhesive strength to the substrate and is porous. Accordingly, it is very difficult to fix the nanowires by providing a patterned binder resin thereon or to form a fixing pattern by irradiating a uniformly applied binder resin with light without impairing the adhesion to the substrate and without damaging the network of the silver nanowires in the conductive layer. Specifically, there is a limitation in accurately providing the patterned binder resin on the conductive layer, and it is also difficult to accurately remove only a non-fixed portion by washing or using an adhesive roller. Also, even in the case where a binder resin is uniformly provided on the conductive layer and a fixing pattern is formed by light irradiation, adhesiveness remains in a portion to which light is not applied, and thus it is difficult to form a precise pattern by accurately and completely removing the non-irradiated portion. In removing the non-fixed portion or the uncured area of the transparent conductive layer thus prepared, in particular, when these patterns are precise and the line width and the distance between adjacent lines are narrow, it is difficult to remove a narrow-space portion and to left a thin-line portion. Consequently, there may be a problem that pattern short-circuit due to incomplete removal of the conductive layer and disconnection due to excessive removal of the conductive layer easily occur in the resulting conductive layer pattern.